Process of making a composite article

ABSTRACT

A process for preparing a composite, continuous-fiber-containing filament which entails: 
     a) unwinding a continuous fiber bundle from a bobbin, 
     b) loosening the fiber bundle, in order to separate the bundle substantially into individual fibers, 
     c) impregnating the individual fibers with a thermoplastic polymer powder, and 
     d) recompacting the fiber bundle and providing a flexible sheltering sheath around the bundle, whereby the fiber bundle is caused to have a substantially elliptical cross-section.

This is a divisional application of Ser. No. 08/242,600 filed on May 13,1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing a composite,continuatuous-fiber-containing filament.

2. Description of the Background

The present invention relates to a flexible, composite, thermoplasticfilament which contains endless (i.e., continuous) fibres and to theprocess for preparing it.

More particularly, the present invention relates to a flexible,composite, thermoplastic filament constituted by fibre bundles ofendless fibres of any kinds, whether mineral, or organic or metalfibres, impregnated with a thermoplastic powder and coated with anexternal flexible, thermoplastic sheath, which may either be of the sametype, or of a different type, as of the thermoplastic component of thepowder.

In a large number of applications, heretofore dominated by the use ofmetal materials, composite materials were proposed which are constitutedby a polymeric matrix and an inorganic reinforcer component, above allfibreglass or carbon fibres. Under conditions under which particularlyhigh performance are required as regards the mechanical strength,long-fibre or, above all, endless-fibre composites are proposed.

The first type of long-fibre or endless-fibre composite, developed as analternative to the metal materials, was characterized by having a matrixconstituted by thermosetting polymers, typical example of which areepoxy resins, unsaturated polyester resins and bismaleimides.Fabrication technologies for processing these type of materials were,and still are, preimpregnation and pultrusion.

Both said technologies require that the fibres are caused to flowthrough suitable solutions of the thermosetting material, withconsequent environmental problems and high process cost, owing to theneed for managing the re-use and/or the storage of the used solvents.

In order to obviate this type of drawbacks, more recently compositematerials were introduced which use thermoplastic polymers as theirmatrix. In this case, the relevant fabrication technology implies, whenthe composite is a long- or endless-fibre one, the preliminaryfabrication of a semifinished intermediate.

An example of preparation of such a type of semifinished intermediate isreported in U.S. Pat. No. 3,742,106; in which the formation of acomposite by impregnation of a continuous reinforcer filament in athermoplastic molten mass is disclosed.

A further example of semifinished intermediate for composite withthermoplastic matrix is reported in Journal of Applied PolymerScience--Applied Polymer Symposium, vol. 47, p. 501, 1991, in which thepreparation is described of an endless filament by spinning a mixconstituted by reinforcer fibres and thermoplastic fibres.

U.S. Pat. No. 4,614,678 discloses the formation of a composite articleby impregnating a fibre bundle with thermoplastic powders and coatingsaid fibre bundle with an also thermoplastic sheath.

SUMMARY OF THE INVENTION

More particularly, according to the process disclosed in this patent:

an endless filament constituted by a fibre bundle, is unwound from abobbin;

the filament is fed to a closed chamber inside which a thermoplasticpolymer powder is kept under a fluidized condition by means of afluidizing gas;

inside said chamber, the filament bundle is loosened in order toseparate it essentially into its individual fibres;

the loosened filament is caused to run through the fluidized polymerduring a long enough time in order that polymer particles can bedeposited around the individual fibres;

the filament is recompacted and around it a flexible sheltering sheathof thermoplastic polymer, preferably of the same kind as of the polymerpowder, is applied.

After obtaining semifinished intermediates, fabricated pieces areproduced by means of consolidating techniques which may require heatingand pressure application, as reported, e.g., under various entries inEncyclopedia of Composites, S. M. Lee editor, published by VCHPublishers, New York, 1990-1991, or pultrusion of said semifinishedarticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-section of a conventional circularcross-sectioned composite filament.

FIG. 2 illustrates a cross-section of the present composite filament.

FIG. 3 illustrates the rhomboidal-shaped cross-section of the tubulardie used in accordance with the present invention.

FIG. 4 illustrates an apparatus for preparing the present composite.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The use of thermoplastic composites undoubtedly allows improvements tobe achieved from the environmental view point because no solvents arerequired by the manufacturing cycles. Unfortunately, however, someproblems arise from the view point of transformation into finishedarticles, above all when the composite material is constituted by acontinuous, multifibre filament impregnated with a thermoplastic polymerpowder. In this case, in fact it was observed that manufacturing thesemifinished intermediate requires extraordinarily long moulding timescaused, as it will be demonstrated hereinafter, by the particularstructure of the material, which appears as a flexible cord having anessentially circular cross section.

The purpose of the present invention is of providing a flexible,composite material which contains endless fibres impregnated with athermoplastic powder, which requires very much reduced moulding times ascompared to those required by the equivalent materials known from theprior art.

The present Applicant found now that the above stated purpose can beachieved if one uses, as the semifinished intermediate, a compositebased on endless fibres impregnated with a thermoplastic powder andcoated with a flexible sheath, having the form of a flexible cord, withan essentially elliptical cross section.

Therefore, the object of the present invention is a flexible, composite,thermoplastic filament, which contains endless fibres and is essentiallyconstituted by a flexible sheath of a thermoplastic polymer which coatsa bundle of fibres impregnated with a thermoplastic powder,characterized in that said filament has an essentially elliptical crosssection with d₁ /d₂ lower than 1, preferably comprised within the rangeof from 0.1 to 0.5, wherein d₁ represents the minor diameter and d₂represents the major diameter of said essentially elliptical section.

The composite fi lament according to the present invention displays ahigh flexibility which makes it possible it to be woven in order toproduce highly flexible mats, or to be used as such in order to producearticles having any shape or size, by means of the so said "filamentwinding" technique. The filament may have a fibre content of up to 70%by volume, and preferably comprised within the range of from 40 to 60%.

Any types of fibre can be used in order to prepare the compositeaccording to the present invention.

Typical examples are fibreglass with a count comprised within the rangeof from 150 to 2500 tex, the aramidic fibres of KEVLAR type, having acount comprise within the range of from 1000 to 2000 decitex, or carbonfibres with a count of from 3×10³ to 12×10³ filaments.

The particle size of the powder which impregnates the fibres is notcritical for the realization of the composite filament according to thepresent invention. The only critical parameter may possibly be thethickness or diameter of the individual fibres which constitute thefibre bundle.

In fact, on considering that the impregnation of the fibre bundle withpowder is yielded by the particles gathering around each fibre, it ispreferable that said particlaes have an average size which isapproximately the same as of, or smaller than, the thickness of eachindividual fibre. In general, the average diameter of the individualparticles is comprised within the range of from 10 to 100 micrometers.

Alternatively to the powder, also thermoplastic endless fibresinterlaced with the reinforcer fibres can be used.

Every thermoplastic polymer can be used in order to prepare thethermoplastic powder which impregnates the composite filament accordingto the present invention, and illustrative examples comprise:polyolefins, such as low-, medium- or high-density polyethylene,polypropylene, polystyrene, ethylene copolymers with propylene and/orbutenes, halogenated polyolefins such as polyvinyl chloride,polyvinylidene chloride, polyvinyl fluoride, polyvinylidene fluoride,styrene copolymers (ABS, SAN, etc), aliphatic (co)polyesters such aspolyethylene terephthalate, polybutylene terephthalate, polyethylenenaphthalene dicarboxilate and their copolymers; aromatic polyesters;polyacrylates and/or polymethacrylates and their copolymers;thermotropic liquid-crystal polymers; Pa-6, Pa-6,6, Pa-4,6, Pa-11 andPa-12 polyamides; polycarbonates, e.g., bisphenol-A polycarbonates;polyether-imides; polyketones; polysulfones; and so forth; and theirmixtures.

The flexible sheath which coats the fibre bundle is present in such away as to have a weight ratio of the impregnating powder to the samesheath comprised within the range of from 1:2 to 2:1.

The material the sheath consists of is selected from the previouslylisted thermoplastic polymers, and can be of either the same type as, ordifferent from, the thermoplastic powder component. Sheaths made frompolymers not only different from, but also having higher or lowermelting temperatures than of the polymers which constitute the powder,can be used as well.

The outer sheath is applied to the impregnated fibre bundle by means ofwell known extrusion techniques, disclosed in European patentapplications published under Publ. Nos. 188,939 and 190,522.

According to these techniques, the powder impregnated fibre bundle iscaused to coaxially run through a tubular die connected with anextruder. The plastic material, extruded through the tubular die, isdragged by viscous effect by the bundle during its passage through thedie and, after a few seconds, cools down, forming said sheath.

In order that the cross section of the composite filament may be givenan essentially elliptical shape, the tubular die has its cross-sectionhaving a geometrical rhombus shape with mean values of its diagonalsapproximately equal to d₁ and d₂.

In FIG. 1 and 2, the cross sections are schematically displayed of thecomposite filaments obtained by means of the methods known from theprior art and according to the present invention, respectively.

The composite (1) comprises a sheath (2) which coats the fibres (4)impregnated with the powder (3).

FIG. 3, the rhomboidal-shaped cross section of the tubular die isdisplayed wherein, by short-dashed line, the perimeter formed by themean diagonals is represented. The die comprises the die nose (5) andthe external jacket (6).

An alternative method in order to produce the composite filament with across section of essentially elliptical shape, provides for thepowder-impregnated fibre bundle to be flattened by a pair of rollers andthe sheaths to be extruded through a tubular die with a circular crosssection of larger diameter than of the maximal thickness of theflattened fibre bundle.

The composite material according to the present invention displays ahigh flexibility which makes it possible it to be woven in order to formhighly flexible mats, or to be used in its pristine state in order tofabricate articles of any shape or size, according to the "filamentwinding" technique.

More particularly, the composite material according to the presentinvention is suitable for being used instead of metal and their alloysin building, transportation, aerospace sectors, and so on.

A process for producing the composite, continuous-fibre-containingfilament according to the present invention comprises:

(a) unwinding a continuous (endless) fibre bundle from a bobbin;

(b) loosening the fibre bundle, in order to separate it essentially intoits individual fibres;

(c) impregnating the individual fibres with a thermoplastic polymerpowder;

(d) recompacting the fibre bundle and providing a flexible shelteringsheath around it, also made from a thermoplastic polymer, by causing thefibre bundle to coaxially run through a tubular die having its crosssection of rhomboidal shape with mean diagonals equal to d₁ and d₂.

The loosening of the fibre bundle can be carried out by means of amechanical system with small-diameter rollers of vibrating type,pneumatic type, and the like. When the fibre bundle is loosened, thelatter is ready for its component fibres to be impregnated with thethermoplastic powder.

Any systems capable of impregnating endless fibres with a powder can beused in the process according to the present invention.

An operable impregnation system can be that fluidized-bed system whichis disclosed in U.S. Pat. No. 4,614,678; according to which the by nowloosened fibre bundle is caused to run through the powder kept underfluidized conditions.

The fluidized bed is constituted by a sealed chamber inside which thepolymer powder is kept fluidized (i.e., suspended) by circulating air ornitrogen.

As reported hereinabove, the size of powder particles is not critical,but said powder particles should preferably have an average size whichis approximately the same as, or lower than, the diameter of eachindividual fibre. In general, the average diameter of the individualparticles is comprised within the range of from 10 to 100 micrometers.

In order to favour the impregnation, the loosened filament is driveninside the interior of the fluidized bed by means of metal pulleys,preferably of aluminum and asimmetrically arranged inside the sealedchamber, with at least one of them being electrically connected withearth. In general, the number of pulleys is comprised within the rangeof from 2 to 6.

According to another impregnation system, the loosened fibre bundle isdipped and caused to run through a stationary bed constituted by thethermoplastic polymer powder, contained in a tank submitted tocontinuous vibration.

By "stationary bed", as this term is used in the instant disclosure andin the appended claims, a powder bed or layer is meant in which theindividual particles can freely move along the horizontal plane, withany vertical motions of the particles being exclusively limited to thethickness of the layer.

The vibrations of the polymer powder containing tank can be obtained bymechanical means, by swinging arms, or by means of an electromagnetichead. The intensity of the vibrations is not very high, but should besuch as prevent preferential channels from being formed in the powderduring the passage of the fibre bundle.

The process for preparing flexible composite materials according to thepresent invention can be better illustrated by referring to the drawingof the accompanying FIG. 4, which represents a non-limitative example ofoperating scheme thereof.

An endless fibre bundle (1) is unwound from a bobbin (2) and is fed to atightly sealed chamber (3) through the sealed opening (4).

Before entering the chamber, the fibre bundle is loosened, andessentially separated into its individual fibres, by means of two ormore rollers (5).

Inside the chamber (3), a thermoplastic polymer powder (6) is kept underfluidized conditions by an air or nitrogen stream fed through the duct(7).

The loosened fibre bundle is pulled to run through the fluidized bedthrough which it is driven by the pulleys (8), (8') and (8") and, duringthis passage, the fibres are coated by the powder.

The powder impregnated fibre bundle leaves the chamber (3) and isrecompacted through the sealed opening (10) above which a tubular die(11), fed by an extruder (12), suitable for coating thepowder-impregnated fibre bundle with a flexible sheath of thermoplasticpolymer. The die (11) is illustrated in greater detail in FIG. 3.

The so prepared composite filament is caused to run around the pulley(13), in order to favour its air-cooling, then, driven by rollers (14),is collected on bobbin (15).

In order to better understand the present invention and to practice it,some illustrative, non-limitative examples thereof are reported in thefollowing.

EXAMPLE 1

A fibre bundle of carbon fibres of 6000 filaments (6K), manufactured bySOFICAR, with a count of 400 g/km is impregnated with a powderconstituted by polyether-ether-ket one (PEEK) 150 P ex Imperial ChemicalIndustries (ICI). Said PEEK was preliminarily ground in order to obtaina powder with particles size of approximately 45 micrometers.

The impregnation is carried out by causing the fibre bundle to run, at aspeed of 45 meters/minute, through a mechanical loosener constituted by4 rollers of 20 mm of diameter and causing the by now loosened fibrebundle to subsequently run through a fluidized bed obtained by injecting1600 liters/hour of nitrogen through 900 g of powder contained inside asealed chamber.

Inside the interior of said sealed chamber, the loosened fibre bundle isguided through three pulleys asimmetrically arranged in such a way as toalways have a filament length of about 0.5 meters dipped inside thefluidized bed.

At the end of the impregnation, the sheath constituted by the samepolymer as the polymer powder, is applied to the fibre bundle.

The sheath is applied to the filament by means of an extruder ofMaillefer 30 type equipped with a tubular die the cross section of whichhas a rhomboidal geometrical shape the mean diagonals of which are inthe ratio of d₁ /d₂ =0.127, and with a surface area of 0.4 mm².

The so prepared composite filament displays a transversal cross sectionof elliptical shape with major and minor diameters of 1 mm and 0.127 mmrespectively.

With the so prepared composite filament, specimens were prepared inorder to determine the moulding times.

The filament was manually wound around a rectangular sheet, caring offilling the available room as evenly as possible.

The sheet was submitted to partial welding in order to favour thehandiness thereof, and then was cut in perpendicular direction to thefilament, in order to have sample units of 20×20 cm of size, available.Then, the sheets were moulded at PEEK melting temperature. The requiredtime for completely consolidating the sample is of the order of 16minutes.

EXAMPLE 2

This test was run by following the previously specified modalities,using a fibre bundle of SOFICAR carbon fibres of 3000 filaments (3K).The resulting composite filament had a cross section with ellipticalshape, with diameters of 0.5 and 0.134 mm, respectively, and a surfacearea of 0.2 mm² . The moulding time was of about 18 minutes.

Using the same materials, but operating according to the principlesdisclosed in U.S. Pat. No. 4,614,678, a composite filament with circularcross section of 0.2 mm² of surface area was prepared. From thismaterial, an analagous specimen to those as of Examples 1 or 2 wasprepared. The moulding time was of about 30 minutes.

We claim:
 1. A process for preparing a composite, continuous-fibre-containing filament, which entails:a) unwinding a continuous fibre bundle from a bobbin, b) loosening the fibre bundle, in order to separate the bundle substantially into individual fibres, c) impregnating the individual fibres with a thermoplastic polymer powder, and d) recompacting the fibre bundle and providing a flexible sheltering sheath around the bundle, the bundle being made from a thermoplastic powder, causing the fibre bundle to coaxially run through a tubular die, whereby the bundle is caused to have a substantially elliptical cross-section with d₁ /d₂ being less than 1, wherein d₁ and d₂ are mean diagonals, with d₁ representing a major diameter and d₂ representing a minor diameter of the substantially elliptical cross-section.
 2. The process according to claim 1, wherein the loosening of the fibre bundle is effected by a means of a mechanical system with vibrating or pneumatic type rollers.
 3. The process according to claim 1, wherein the impregnation of the fibre bundle is carried out by causing the loosened fibre bundle to run through the powder kept under fluidized conditions inside a sealed chamber.
 4. The process according to claim 1, wherein the impregnation of the fibre bundle is carried out by dipping and pulling the loosened fibre bundle through a stationary bed constituted by a thermoplastic plastic polymer powder contained inside a tank submitted to continuous vibration.
 5. The process according to claim 1, wherein said filament has a fibre content of up to 70% by volume.
 6. The process according to claim 1, wherein the powder impregnating the fibres comprises particles having an average diameter of from 70 to 100 μm.
 7. The process according to claim 1, wherein the flexible sheath which coats the fibre bundle is present in such a way as to have a weight ratio of the impregnating powder to the same sheath comprised within a range of from 1:2 to 2:1.
 8. The process according to claim 1, wherein the sheath is applied to the impregnated fibre bundle by co-axial extrusion through a tubular die the cross-section of which has a geometrical shape of a rhombus and having mean diagonals approximately equal to d₁ and d₂.
 9. The process according to claim 1, wherein the continuous-fibre-containing filament, comprises endless fibres and consists essentially of a flexible sheath of a thermoplastic polymer which coats a fibre bundle of fibers impregnated with a thermoplastic polymer.
 10. The process according to claim 5, wherein said filament has a fibre content of from 40 to 60% by volume.
 11. The process according to claim 1, wherein said fibre is selected from the group consisting of fibreglass with a count of from 150 to 2,500 tex, aramidic fibres having a count within the range of from 1,000 to 2,000 decitex and carbon fibres having a count of from 3×10³ to 12×10³ filaments.
 12. The process according to claim 1, wherein said thermoplastic powder comprises particles, an average diameter of which is within the range of from 10 to 100 μm.
 13. The process according to claim 1, wherein d₁ /d₂ is within the range of from 0.1 to 0.5.
 14. The process according to claim 1, wherein said flexible sheltering sheath is made of either the same or different material than said thermoplastic powder. 